Circling Back To Littles Law Now that we have tools to gather information.

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Presentation on theme: "Circling Back To Littles Law Now that we have tools to gather information."— Presentation transcript:

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Circling Back To Littles Law Now that we have tools to gather information

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2 Latency – A measure of time delay experienced in a system, the precise definition of which depends on the system and the time being measured. In storage, latency is generally referred to as response time, in ms. Throughput – The amount of material or items passing through a system or process. In storage, IO/s in units of 4k

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8 Littles Law - Review We can use this with Latency & Throughput on a Netapp system too. Standard version: Re-written for Netapp: Translating into IO terms: N = # of outstanding IOs T = Throughput of IOs R = Response time of each IO

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9 Littles Law - An Example Typical situation: – An user complains of poor performance: My dd/cp/tar/Oracle query (for example: full table scan) etc. process isnt fast enough – A casual look at sysstat shows the filer is not very busy – NetApp Service returns with a statement of thread-limited What does this mean?

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Littles Law - An Example Compute Wait for Storage Read Request Data Return Time In this example, the process is either computing or reading. It is always busy. But the CPU and the storage are not, on average, fully used. Client side tools would be needed to determine this: debugger, strace, dtrace, etc.

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14 Littles Law - An Example This example is a concurrency-limited workload – Each thread is always busy – Not enough threads to keep the system busy Implications: – Storage system not fully utilized – High I/O wait times at the server